The identification of chemokine receptors as coreceptors for HIV entry, not only has contributed to the understanding on viral tropism but has provided an additional target for therapeutic intervention for HIV disease. Several chemokine receptors have been shown to function as coreceptors for HIV-1 entry. The main ones are CXCR4 (for T-cell line tropic viruses) and CCR5 (for macrophage-tropic viruses). Because of the capacity of HIV to adapt when selective pressures are imposed, it is likely that any drug designed to block the interaction of HIV with one coreceptor will force the virus to use additional coreceptors. Thus, the determination of the complete coreceptor repertoire is necessary. Because STRL33 is expressed in all lymphoid tissues, in collaboration with Dr J. Farber, NIAID, we tested it for coreceptor activity with HIV. STRL33 expression in Jurkat cells conferred increased permissivity to infection by the ELI1 isolate of HIV-1. Thus, STRL33 can act as an HIV-1 coreceptor in vitro. As well as testing the coreceptor activity of STRL33 with a number of HIV-1 strains of different phenotypes, we have begun studies with HIV-2 and SIV. We have shown, in an infectivity assay, that the MAL strain of HIV-1 and the mac239 isolate of SIV use STRL33 but not as well as they use CCR5. The appearance of virus only after about 30 days in culture is indicative of adaptation. To confirm this, virus emerging after about 35 days was used to infect fresh Jurkat- STRL33 cells as well as the parent Jurkat cells. In this second passage, virus production was seen after about 12 days, thus demonstrating that both SIVmac239 and HIV-1 MAL had adapted to use STRL33 more efficiently. Importantly, these passaged viruses were still unable to infect Jurkat cells. That the passaged virus had adapted to use STRL33 was demonstrated by the fact that an antibody raised to STRL33 inhibited virus infection. We have cloned 12 envelope genes from the adapted MAL virus and 6 env genes from the adapted SIVmac239. We are currently returning these genes to the infectious molecular clones to assess their biological activities. Based on the cell tropism of a number of viruses, both SIV and HIV-1, we predicted the existence of another coreceptor on SUP-T1 cells. We have cloned this coreceptor and identified it as CCR9, a chemokine receptor for TECK (CCL25). CCR9 exists in two forms, A and B, that differ by 12 amino acids at the N terminus. CCR9B has activity with several strains of SIV, including SIVagmSAB and SIVagmTAN, but as yet no HIV-1 has been found that uses this molecule as an entry cofactor. Additional HIV-1 isolates including primary isolates are being screened. Since CCR9A/B is expressed in thymocytes and in the lymphocyte subset that trafficks to the gut, and because of the obvious importance of these tissues to HIV disease and AIDS pathogenesis, we are expanding our studies to include the coreceptors expressed in these cells as well as to the infection of these cells and of dendritic cells, which secrete CCL25 and thus may recruit T cells.